1. Field of the invention
The present invention relates to a laminate printed circuit board having leads for plating.
2. Description of the Background Art
In parallel with a decrease in the size and thickness of memory ICs (Integrated Circuits), memory modules for mounting them are also decreasing in size and thickness. While an SIMM (Single Inline Memory Module) is one of the conventional miniature memory modules, a DIMM (Dual Inline Memory Module) is highlighted today. The SIMM has external terminals arranged on its front and rear surfaces and electrically connected by through via holes. By contrast, in the DIMM, a particular signal flows through each of the front and rear external terminals adjoining each other. The through via holes, or conduction paths, are formed in a laminate printed circuit board in order to provide electrical conduction between different layers. Specifically, the via holes may be formed throughout the circuit board in order to electrically connect the front and rear of the circuit board. Alternatively, the via holes may be so formed as to connect the surface layer of the circuit board and an intermediate layer present in the circuit board, or nearby intermediate layers.
FIGS. 2A and 2B show a specific configuration of the conventional SIMM or DIMM. As shown in FIG. 2A, leads 3 for plating terminals 2 are led out from the end face of the circuit board. This brings about a problem that, as shown in FIG. 2B, when the leads 3 are cut off by a rooter after the plating of the terminals 2, their residuals 4 are deformed and are apt to short-circuit nearby terminals 2. There are also shown in FIGS. 2A and 2B through holes 1 and a side face 5 cut by a rooter.
It is therefore an object of the present invention to provide a laminate printed circuit board having leads for plating, and capable of eliminating the above problems and reducing the wiring area.
In accordance with the present invention, a laminate printed circuit board includes a conductor provided on each of the front and rear of the circuit board. A wiring is connected to the conductor. A a blind through hole (BTH) is connected to the wiring. A lead for plating forms the intermediate layer of the circuit board and is connected to the BTH. The BTH refers to a through hole for electrically connecting two different layers of the circuit board; at least one of the two layers is an intermediate layer. The BTH therefore does not extend throughout the front and rear of the circuit board. Because the lead forms the intermediate layer, it does not turn out a deformed residual when cut off by a rooter after the plating of the conductor. Further, when the present invention is applied to, e.g., a DIMM, it reduces the wiring area, as will be described hereinafter.
As shown in FIG. 5A, to obviate the deformed residual, leads 6 may be led out by way of through via holes 7 in the case of an SIMM. This kind of scheme, however, is not desirable when it comes to a DIMM for the following reason. As shown in FIG. 5B, in the case of a DIMM, an exclusive through via hole extends from each terminal because the front and rear electrodes are different from each other. As a result, the through via holes are concentrated with twice the density than the via holes of the SIMM in the same area. This makes wiring design difficult or renders the wiring practically impossible.
The present invention is capable of forming, with BTHs, leads for wiring even with a DIMM or similar module in which the front and rear terminals each allows a particular signal to flow. This halves the area of the through holes, compared to the case with the through via holes.
Also, in accordance with the present invention, a laminate printed circuit board includes a plurality of conductors provided on the circuit board and to be plated. A plurality of wirings are respectively connected to the conductors. A plurality of BTHs are respectively connected to the wirings. A lead for plating forms the intermediate layer of the circuit board and connects the BTHs. After the conductors have been plated via the lead, the lead is cut off between nearby BTHs to thereby electrically isolate the nearby BTHs.
Because nearby BTHs are connected at the intermediate layer so as to implement the lead at a time, the lead is prevented from turning out the deformed residuals. As for a DIMM, such a configuration reduces the wiring area. In addition, a vacant portion for accommodating a ground (GND) pattern is available on the intermediate area below the terminal. The GND pattern ensures a constant impedance at the terminals portions wherein signals influence each other in a complicated way, thereby reducing noise.
The impedance at the terminal portions will be described specifically with reference to FIG. 6. As shown, terminals 21 are provided on the surface of a laminate printed circuit board 40 while a GND pattern 15 is formed on an intermediate layer below the terminals 21. Each terminal 21 and ground pattern 15 forms an impedance 61 therebetween. Why the GND pattern ensures a constant impedance is as follows.
Assume that each terminal 21 has a thickness of t and a width of w and is spaced at a distance of h from the GND pattern 15, and that the material intervening between the terminal 21 and the GND pattern 15 has a specific dielectric constant and an effective specific dielectric constant of ∈r and ∈r e, respectively. Then, the impedance 61 has a size Zo expressed as:
Zo=(60/{square root over (∈r e+L )})xc2x7(ln(5.98h/(0.8w+t))
As the above equation indicates, the impedance 61 can be maintained constant if the distance h between the terminal 21 and the GND pattern 15 is constant. For example, if the GND pattern 15 is uniformly spaced from the terminals 21, the impedance 61 remains constant. The constant impedance 61 successfully absorbs noise waves and allows a minimum of current causative of malfunction to flow through the terminals 21.
Further, in accordance with the present invention, a laminate printed circuit board includes a plurality of conductors provided on the circuit board and to be plated. A lead for plating connects the conductors. After the conductors have been plated via the lead, the lead is cut off between nearby conductors to thereby electrically isolate the nearby conductors. In this case, a GND area may also be provided in order to maintain the impedance at the terminal portions constant for the above purpose. Moreover, this configuration eliminates the need for BTHs and can be implemented by a conventional simple technology. In addition, because the wirings are formed and cut off on the surface of the circuit board, easy repair is promoted at the stage of visual test.